In recent years, the use of wireless communication systems having mobile transceivers which communicate with a hardwired network, such as a local area network (LAN) or a wide area network (WAN), has become widespread. The mobile transceivers, commonly referred to as mobile terminals, may take one of several different forms. For instance, in retail stores hand-held scanning units may be used to allow for scanning inventory bar codes. In a warehouse, portable units mounted to a vehicle may be used to gather information from the warehouse floor. In a medical environment, the mobile terminal may take the form of a pen based workslate which allows medical personnel to work with full page screens at once.
In a typical wireless communication system, each mobile terminal communicates with a networked system via a radio or optical link in order to allow for a real time exchange of information. The mobile terminals communicate through one of several base stations interconnected to the network. The base stations allow for a wireless data communication path to be formed.
Each mobile terminal and base station communicate via their respective transmitter and receiver (i.e., transceiver) systems. Typically, the transmitter and receiver in each device share the same antenna and a control signal is used to switch the antenna between a transmitting and receiving mode. Thus, only one of the transmitter or receiver needs to be active at any given time.
Information exchanged between mobile terminals and base stations is generally sent in packet format. Packets of information (also referred to herein simply as "packets" or "data packets") are a defined set of data bits which carry information such as source address, destination address, synchronization bits, data, error correcting codes, etc.
In order to provide for an efficient operating system, access to a channel is expeditiously controlled by a media access protocol. For example, a typical media access protocol may provide that if a device receives a packet which requires a response, transmission of the response must be initiated within a time period on the order of 15 micro-seconds after receiving the entire original packet. This time span, or slot time, is referred to as the short inter-frame space (SIFS). The purpose of the SIFS is twofold. First, it speeds up data exchange between devices by limiting the amount of time a device can take to respond. Secondly, it limits the amount of time a device receiving the response must remain in the receiving mode. As mentioned above, since the transmitter and receiver oftentimes are connected to the same antenna, a device may miss information if it is transmitting information at the same time it could be receiving information.
During such time when a device is receiving information but not transmitting information, a fully powered transmitter can nevertheless consume a considerable amount of power. In order to conserve power in the above mentioned mobile terminals, for example, the transmitter and perhaps other non-essential circuitry is generally placed into a sleep-like state, referred to herein as a "sleep mode", during periods where the system is receiving information. During the sleep mode, the power provided to the transmitter and other non-essential circuitry is reduced to minimum levels. Since most mobile terminals operate on battery power, the sleep mode helps maintain a longer usable battery life without the need for recharging or replacing the battery.
An unfortunate consequence of placing the transmitter of a device into a sleep mode is that the data exchange rate will be reduced. The reduction in data exchange rate is attributable to the time required for the transmitter to stabilize into a fully powered or active state after a signal is sent to the transmitter indicating that it needs to transmit information. For instance, it is common for transmitter circuitry to take approximately two-thirds of the allowed SIFS time to stabilize. During such time, information is neither being transmitted or received by the responding device. As a consequence, overall data exchange rate suffers. Furthermore, given the strict SIFS time limitations typically in place, it is difficult for the transmitter to respond to a packet during the short period of SIFS time remaining after the transmitter has reached a fully powered state (i.e., stabilized). Systems attempting to meet the aforementioned strict SIFS time limitations typically have higher probabilities of errors occurring in each transmission. In addition, as higher spectral density modulation techniques are implemented (typically to increase data rate), more complex transmitter circuitry is utilized which requires even longer stabilization time. Thus, even greater possibilities of transmission errors or slower data exchange rates exist.
It is also known in the art to conserve power within a device by reducing the power provided to the receiver when a device is transmitting information. According to one conventional approach, a transceiver in a mobile terminal powers up its receiver only at predetermined times or intervals during which the device may receive information. For the remainder of the time, the receiver circuitry remains in a powered down state, i.e., a sleep mode. For example, according to one conventional protocol, during the times the receiver of the mobile terminal is powered up, the mobile terminal listens for "beacons" sent from base stations indicating there is information which needs to be transmitted to the mobile terminal. If information within a beacon indicates that a base station has information stored therein to be transmitted to the mobile terminal, the mobile terminal in turn transmits a "poll" packet requesting that the information be sent. By using this protocol, the mobile terminal can keep its receiver in a sleep mode at all times except when it is active to listen for a beacon and for a short time after the mobile terminal sends a poll packet and is therefore poised to receive information buffered in the base station. Thus, power may be conserved.
Unfortunately, however, regardless of when a base station is prepared to communicate with the mobile terminal, the base station must buffer all information until such time when the mobile terminal indicates to the base station that its receiver is activated from the sleep mode to receive information. The undesirable result of this power saving protocol is that, in exchange for the power conservation obtained via this mode, a substantial reduction in the data exchange rate results. More specifically, since the mobile terminal receiver cannot receive information at all times there is a delay in the exchange of data.
Other conventional approaches for conserving power by placing the receiver into a sleeping mode suffer from similar drawbacks. For example, according to another technique the mobile terminal and base station are configured in a master-slave relationship. The mobile terminal is designated master and powers up its receiver from a sleep mode only during such times as the mobile terminal expects/desires to receive information. Again, however, the mobile terminal cannot receive information at all times. As a result, the data exchange rate is much lower than in the case where the mobile terminal is able to receive information at virtually any time.
In view of the aforementioned shortcomings in existing transceivers due to the combination of the necessity to conserve power, the time limitations for responding to an information packet, and the desire for higher data rates, there is a strong need in the art for a transceiver which overcomes such drawbacks. More specifically, there is a strong need in the art for a transceiver in which the switching of the transmitter between an active mode and a sleep mode allows for sufficient time to respond to packets within a prescribed response period, even at high data rates in which the system may be operating. Furthermore, there is a strong need for a transceiver having a receiver which can receive data packets at any time while still utilizing a low power consumption mode of operation. More generally, there is a strong need in the art for a transceiver and system which can maintain an optimal data exchange rate while continuing to conserve power by operating in a power savings mode of operation.